from Mere Christianity by C. S. Lewis

I freely admit that real Christianity (as distinct from Christianity-and-water) goes much nearer to Dualism than people think. One of the things that surprised me when I first read the New Testament seriously was that it talked so much about a Dark Power in the universe—a mighty evil spirit who was held to be the Power behind death and disease, and sin. The difference is that Christianity thinks this Dark Power was created by God, and was good when he was created, and went wrong. Christianity agrees with Dualism that this universe is at war. But it does not think this is a war between independent powers. It thinks it is a civil war, a rebellion, and that we are living in a part of the universe occupied by the rebel. Enemy-occupied territory—that is what this world is. Christianity is the story of how the rightful king has landed, you might say landed in disguise, and is calling us all to take part in a great campaign of sabotage. When you go to church you are really listening-in to the secret wireless from our friends: that is why the enemy is so anxious to prevent us from going. He does it by playing on our conceit and laziness and intellectual snobbery.
Lewis, C. S. (2009-05-28). Mere Christianity (p. 46). Harper Collins, Inc.. Kindle Edition.